Most conventional piezoresistive pressure sensors employ the full bridge principle, i.e. an entire Wheatstone bridge, comprising four resistors, is located on a membrane. There are certain advantages with such a solution, and some disadvantages. The advantages are that they have a high sensitivity, little temperature sensitivity, and a low off set.
The major disadvantage is that they are bulky and thus impose a lower limit on the dimensions of the sensor.
Another major problem associated with all pressure sensors, and in particular with ultraminiature pressure sensors, is to manage to manufacture identical elements in all batches over time. It is virtually inevitable that the sensor characteristics differ from sensor to sensor and hence there will always be a need for some kind of individual calibration of each sensor. Typically it is necessary to calibrate the output signal for a change in temperature, i.e. the pressure response of each sensor will have individual and varying degrees of temperature dependence.
In European patent application 85100922.5 (corresponding to U.S. Pat. No. 4,734,873) there is disclosed to store the sensor characteristics in a PROM and to utilize a computer for the calculation of the calibrated signal.
Another alternative used for piezoresistive pressure sensors is to integrate a resistor net on the chip or in the vicinity of the chip, in order to normalize the signals.
Sensors from CAMINO and other pressure sensor devices comprise components that have been individually matched for the sensor element in order to calibrate the sensor device.
U.S. Pat. No. 5,551,301 (Cardiometrics) discloses a method of calibrating a sensor using two amplifiers and a computer.
An important problem associated with the ultraminiature devices comprising additional resistors and/or other components, such as PROMs, provided on the device, is the small dimensions of the sensor. The mentioned components are simply too large to be easily integrated in a sensor element, if the necessary degree of miniaturization is to be achieved. There may also be required too many connection leads between the sensor and the electronic circuitry, which prevents the miniaturization. The higher the number of leads is, the closer to each other the leads have to be placed, and thus the higher the risk for short-circuit and damage on the leads will be.
Another important aspect of employing guide wires, is that it is necessary to be able to disconnect the guide wire from external devices. This is because for e.g. dilatation purposes, after having measured pressure and identified a stricture, it will be necessary to insert a balloon catheter in the coronary vessel. This catheter is inserted by threading it into the guide wire, and using the guide wire to pass the balloon to the desired site of dilatation. Therefore the guide wire and an interface cable must be connectable to and disconnectable from each other in order to allow this insertion of a balloon catheter.